This invention relates to underfill encapsulant compounds prepared from epoxies to protect and reinforce the interconnections between an electronic component and a substrate in a microelectronic device. Microelectronic devices contain multiple types of electrical circuit components, mainly transistors assembled together in integrated circuit (IC) chips, but also resistors, capacitors, and other components. These electronic components are interconnected to form the circuits, and eventually are connected to and supported on a carrier or a substrate, such as a printed wire board. The integrated circuit component may comprise a single bare chip, a single encapsulated chip, or an encapsulated package of multiple chips. The single bare chip can be attached to a lead frame, which in turn is encapsulated and attached to the printed wire board, or it can be directly attached to the printed wire board.
Whether the component is a bare chip connected to a lead frame, or a package connected to a printed wire board or other substrate, the connections are made between electrical terminations on the electronic component and corresponding electrical terminations on the substrate. One method for making these connections uses polymeric or metallic material that is applied in bumps to the component or substrate terminals. The terminals are aligned and contacted together and the resulting assembly is heated to reflow the metallic or polymeric material and solidify the connection.
During its normal service life, the electronic assembly is subjected to cycles of widely varying temperature ranges. Due to the differences in the coefficient of thermal expansion for the electronic component, the interconnect material, and the substrate, this thermal cycling can stress the components of the assembly and cause it to fail. To prevent the failure, the gap between the component and the substrate is filled with a polymeric encapsulant, hereinafter called underfill or underfill encapsulant, to reinforce the interconnect material and to absorb some of the stress of the thermal cycling. In addition, the material helps to absorb impact energy and improve so-called “drop test” performance.
Two prominent uses for underfill technology are for reinforcing packages known in the industry as chip scale packages (CSP), in which a chip package is attached to a printed wire board, flip-chip ball grid array (BGA) in which a chip is attached by a ball and grid array to a printed wire board, and flip chip devices.
In conventional capillary flow underfill applications, the underfill dispensing and curing takes place after the reflow of the metallic or polymeric interconnect. In this procedure, flux is initially placed on the metal pads on the substrate. Next, the chip is placed on the fluxed area of the substrate, on top of the soldering site, for the case of a metallic connection. The assembly is then heated to allow for reflow of the solder joint. At this point, a measured amount of underfill encapsulant material is dispensed along one or more peripheral sides of the electronic assembly and capillary action within the component-to-substrate gap draws the material inward. After the gap is filled, additional underfill encapsulant may be dispensed along the complete assembly periphery to help reduce stress concentrations and prolong the fatigue life of the assembled structure. The underfill encapsulant is subsequently cured to reach its optimized final properties.
The no-flow underfill process provides a more efficient procedure than that described above for attaching electronic components to a substrate and protecting the assembly with an underfill encapsulant. In the no-flow encapsulation process the flux is contained in the underfill which is applied to the assembly site prior to the component placement. After the component is placed, it is soldered to the metal pad connections on the substrate by passing the full assembly, comprising the component, underfill and substrate, through a reflow oven. During this process the underfill fluxes the solder and metal pads, the solder joint reflows to form the interconnect joint between the component and the substrate, and the underfill cures. Attempts to use fillers in no flow underflow compositions have generally failed because the particles can get between the pad and the solder and interfere with the solder joint formation. Thus, the separate steps of applying the flux and post-curing the underfill are eliminated via this process. As soldering and cure of the underfill occur during the same step of the process, maintaining the proper viscosity and cure rate of the underfill material is critical in the no-flow underfill encapsulation process. The underfill must remain at a low viscosity (in the range of about 3,000–6,000 cps) for ease of dispensing and to allow melting of the solder and the formation of the interconnections between the component and the substrate. It is also important that the cure of the underfill not be unduly delayed after the cure of the solder. It is desirable that the underfill in the no-flow process cure rapidly in one reflow pass after the melting of the solder. Further, the underfill must provide excellent adhesion to the solder mask, solder, and the passivation layer of the die. Overall, the underfill must also exhibit a long work life at room temperature and have good reliability during thermal cycling. No flow underfill materials, however, are relatively rigid and brittle in nature and eventually crack and delaminate during thermal cycling between −55° C. and 125° C.